We have a agreement with the WTO. The WTO says " pay your farmers to 'conserve'"). It ends up in the farm bill.
Under current WTO rules, for any particular year, countries are obliged to maintain support across all agricultural commodities below a set level. The U.S. ceiling for amber box support is $19.1 billion. In addition to green box programs, trade-distorting support that is below a de minimis level (5 percent of the value of production for any particular product, or 5 percent of the value of total agricultural production for non-product specific programs) is not counted towards the annual ceiling. However, once support exceeds the 5 percent level, it all counts toward the amber box ceiling – in effect, countries can have a product-specific de minimis program or an amber box program, but not both. Additionally, support under programs that meet criteria for production-limiting programs, so called blue box programs, is also excluded from the annual calculation against the amber box ceiling.
Source (.pdf)